If you want to “convert” portfolio categories to shop items I’d suggest to use Simple Paypal Shopping Cart, WP eStore or Cart 66 because they integrate seamlessly into most themes – you simply need to add a shortcode to any post/page/portfolio entry page and you’re ready to sell the item. WooCommerce is a very powerful shop plugin but it doesn’t integrate well in “standard” themes which do not support WooCommerce out of the box. I’d use one of the other plugins instead. I’d rather not use Shopp (a friend told me that it doesn’t work well). You can also try Tribulant Shopping Cart Plugin but it just supports posts/pages and no custom post types (eg portfolio entries) and imho it’s not the best choice in your case.